Image display devices such as cathode-ray tube (CRT) display devices, liquid crystal display devices (LCD), plasma display panels (PDP), electroluminescent display devices (ELD), electronic paper displays, tablet PCs, and touch panels are commonly provided with an optical layered body for antireflection on the outermost surface.
Such optical layered bodies for antireflection suppress reflection of images and decrease the reflectance by scattering or interference of light.
A known optical layered body for anti-reflection is an antiglare film including an antiglare layer having surface roughness on a transparent substrate. The surface roughness of the antiglare film scatters external light, thereby preventing a decrease in visibility due to reflection of external light and reflection of images. Such an optical layered body is usually disposed at the outermost surface of an image display device, and therefore needs to have hard coating properties in order to prevent the device from being scratched during handling.
A known conventional antiglare film is a film including a light-transmitting substrate and an antiglare layer formed on the surface of the substrate by applying a resin containing a filler such as silicon dioxide (silica) (see Patent Literature 1 and Patent Literatures 2, for example).
Examples of the antiglare film include a film in which surface roughness on an antiglare layer is formed by agglomeration of particles such as cohesive silica; a film in which surface roughness on a layer is formed using a resin containing an organic filler; and a film in which surface roughness is transferred on the surface of a layer by laminating a film with projections and depressions.
Such conventional antiglare films all have a light diffusion function and an antiglare function due to the effect of a surface shape of the antiglare layer. In order to improve such antiglare properties, the surface roughness needs to be steep and dense. However, if the surface roughness is steep and dense, the haze of a coating increases to cause white muddiness, which results in a decrease in contrast of display images.
Further, the conventional antiglare films generate a flicker, so-called scintillation, on the surface, problematically decreasing the visibility of a display screen. Scintillation is a phenomenon that minute nonuniformity of brightness appears on a screen surface when an image display device is turned on and transmitted light from the rear side reaches the screen, and the position of the nonuniformity seems to shift when a viewer changes the viewing angle. The phenomenon is particularly noticeable in the case of full white display and full green display.
Particularly in recent years, a demand for high contrast has been raised because of an increase in occasions to see high-resolution images, such as movies, at home. An exemplary known means to meet the demand is an antiglare film prepared by layering a hard coat layer and an antiglare layer (see Patent Literature 3, for example). Such a layered anti-reflection film suppresses scintillation and white muddiness while maintaining the hard coat properties and antiglare properties by smoothing fine projections and depressions on the surface of the antiglare layer using the same resin as that used for the hard coat layer and thereby making the projections and depressions have less undulation and wider intervals. However, such a film has a thickness of 10 μm or more, and thus fails to sufficiently satisfy the recent demand for reducing thickness of antiglare films.
In a case where surface roughness on a layer is formed using organic fine particles or inorganic fine particles solely, thinning of an antiglare film may cause a case where excessive fine particles are disposed in an upper part of the antiglare layer or the fine particles may be agglomerated in the height direction. As a result, the surface roughness becomes too rough and scintillation or white muddiness is caused. If the average particle size of the organic fine particles or inorganic fine particles is reduced to moderate the surface roughness for the purpose of solving the above problem, the surface roughness tends to be rather too gentle, and the antiglare properties are likely to be deteriorated. Consequently, high quality products cannot be stably obtained. Accordingly, it has been desired to provide an antiglare film including an antiglare single layer with a moderate surface roughness capable of sufficiently suppressing scintillation and white muddiness in a bright room and excellent in contrast in a dark room while maintaining hard coating properties and antiglare properties.